Copolymers of ethylene and vinyl acetate (EVAC) containing between 20 and 50 mole % ethylene are of interest because they are precursors to corresponding ethylene/vinyl alcohol copolymers which have outstanding gas barrier properties for use in packaging applications. Because these applications involve extrusion and molding of the polymer, it is very important to control the molecular weight distribution to minimize variations in processability. In order to produce a uniform product it is therefore preferred to prepare the EVAC in a continuous process.
Most copolymers of ethylene and vinyl acetate containing greater than 50% vinyl acetate are prepared by emulsion polymerization. However, solution polymerization produces polymer that is better suited as a precursor to ethylene-vinyl alcohol copolymer for barrier applications. The use of a CSTR to carry out a continuous solution polymerization is well-known, but in order to maintain a steady-state condition in such a reactor several problems must be overcome. By definition a steady-state condition requires that all variables must be held constant, including temperature and concentrations of all reactants and products.
The copolymerization of vinyl acetate and ethylene is a reaction that is particularly difficult to control because the desired operating state is an unstable one with respect to ethylene concentration. If an upset occurs, the system does not react by returning to the original state or by going to a new state slightly displaced from the original operating condition. Instead, the system begins to drift with the result that the conversion, molecular weight, and composition all move away from the desired values. This is a result of the complex relationships between composition, rate of polymerization, and molecular weight. For example, as the proportion of ethylene decreases, the overall rate of polymerization increases and the molecular weight increases. Together these factors result in a higher degree of conversion and a more viscous reaction medium. These conditions result in a further rate increase and higher molecular weight polymer. Thus, a cycle is begun which results in a continuing drift in molecular weight and degree of conversion. This problem is further compounded by the fact that as the viscosity increases the rate of diffusion of ethylene into the reaction medium is slowed, and at some point the rate of diffusion of ethylene into the reaction mixture becomes slower than the rate of polymerization. When this occurs, the control problem becomes much more severe since the composition also begins to drift, causing further upset to the system. Therefore, it is imperative to ensure that the introduction of ethylene into the reaction mixture does not become limited by solution viscosity such that compositional drift begins to destabilize the system. The ethylene concentration can also drift if another gas is introduced into the system. For example, if nitrogen is used to pressure feed streams, nitrogen will be introduced into the reactor and contribute to the pressure. If the total pressure is held constant, the ethylene pressure will decline as the nitrogen pressure increases.
This invention provides techniques to prevent drift in ethylene composition in the reaction mixture. The first technique concerns a method of handling the ethylene with the vinyl acetate by premixing to ensure a constant ratio of the two monomers in the reaction mixture. Another technique concerns a method to ensure that the equilibrium concentration of ethylene is not altered by a buildup of inert gas introduced through the feeds. Continuous processes for making ethylene-vinyl acetate copolymers in solution are known. For example, see Ch. Abs. 100:86672p. Such processes may use methanol as a solvent and be operated at a temperature of 60.degree. C. and a pressure between 550 and 700 psi. The initiator is a radical initiator such as azobis(isobutyronitrile) (AIBN). Patents of interest include U.S. Pat. No. 3,847,845.